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At the time of its introduction by Intel in 1995, the ATX form factor was the most
significant change in system design since the invention of the PC over a decade earlier.
Although it took several years to "catch on", the ATX form factor and its
variants are now the standard in a large segment of the marketplace. In addition, the NLX motherboard and case
form factor--designed to replace LPX--intentionally use the same power supply because
Intel wanted to avoid having another power supply form factor on the market. Therefore,
the ATX form factor is sometimes called the "ATX/NLX" form factor.

On the outside, the ATX power supply appears virtually identical to an LPX power supply
in terms of its dimensions and component placement. The biggest visible difference between
the two is that the power pass-through outlet for the
monitor has been removed (primarily because modern monitors always come with their own
power cord these days, so the pass-through hasn't been commonly used for some time.)

Diagram of the side and rear views of an ATX/NLX form
factor power
supply, with approximate dimensions. Note that the dimensions are
essentially unchanged from those of the LPX form factor. The biggest
change is the removal of the monitor power pass-through connector
from the standard. By the time ATX was introduced these were rarely
used by modern PC monitors in any event.

The inside of the ATX form factor, however, is an entirely different story.
The ATX power supply design differs from the previous market standards, the Baby AT and LPX form factors, in
several important ways:

True Standard: The ATX form factor is a standard, as opposed to the
"de facto standards" of prior form factors. You can find detailed specifications
about ATX and other newer form factors at the Platform Development Support
Web Site. Included there is a document specific to the ATX power supply.

+3.3 V Power: ATX systems were the first to include +3.3 V power directly, avoiding the need for voltage
regulators to provide it on the motherboard.

Soft Power: ATX systems were the ones where the +5 Standby and Power On signals were introduced. These
signals are used along with a change to the way the power
switch works, as part of the "Soft Power" feature that enables features such
as allowing the operating system to turn off the PC.

Changed Motherboard Connectors: Breaking with 15 years of tradition
created by the PC/XT, AT, Baby AT and LPX form factors, Intel specified new motherboard
connectors for the ATX form factor. This was in part due to the additional signals used by
the ATX power supply and motherboards. For compatibility, some motherboards include both
the new and old style of connector. Read more about the
motherboard connectors here.

Modified Fan Direction and Placement: One of the goals of the original
ATX specification was to change the way the power supply fan
worked. At around the time ATX was introduced, cooling fans were becoming the standard for
the newer, faster CPUs on the market. Instead of exhausting air out the back of the case
as had always been the norm, Intel wanted to use this exhaust air to cool the processor
directly, saving the cost of a cooling fan. Therefore, the ATX specification calls for the
fan to run in the opposite direction and be placed near the CPU's location on the
motherboard, to blow on it for cooling. The other advantage of this method is that it
keeps the system cleaner, since air entering the case all comes from one place, and can be
filtered if necessary.
Unfortunately, while a good idea, this hasn't worked out quite the way Intel hoped. The
primary problem is that newer CPUs continue to generate more and more heat as they get
faster, and a regular power supply fan doesn't have enough flow to cool them properly.
This problem is compounded by the fact that the air blowing on the CPU is warmed by the
components in the power supply itself, so it is several degrees above ambient temperature
before it ever gets near the CPU. Thus, newer versions of the ATX specification make the
fan direction optional. The newest ATX power supplies have gone back to the old style of
placing the fan on the back of the power supply and exhausting air to the outside.

Rear view of an ATX power supply, showing its
motherboard and drive connectors. This power supply
blows air out of the system; the vents on the side
are for drawing air from the inside of the system case.

Since it has become the industry standard, ATX power supplies are found everywhere.
Ostensibly designed to work with ATX cases and ATX (and Mini-ATX) motherboards, ATX power
supplies are also used in NLX systems, as mentioned above. They can also be used for
microATX motherboards in microATX cases if the case is large enough, because the ATX and
SFX main motherboard connectors are essentially the
same.